Various studies have conventionally been made concerning so-called "judder" which occurs in drum brakes for a vehicle at the time of braking and thereby generates noise, and many inventions and designs have been produced as measures against "judder."
They are classified, for example, into the following categories: 1) in which the rigidity of the brake shoe is enhanced so as to attain a natural frequency shifted from the frequency at which "judder" is generated; 2) in which the material of the brake lining is changed; 3) a weight (damper weight) is secured to a part of the brake shoe where a large amplitude is exhibited at the time when "judder" is generated; and 4) in which, as disclosed in Japanese Utility Model Publication No. 5-27711, a friction material is pressed against the inner peripheral surface of the brake shoe so as to relatively slide in the circumferential direction of the shoe at the time when "judder" is generated, thereby consuming the vibration energy of the shoe.
According to the recent studies about "judder," it has been considered suitable for the mechanism for generating "judder" to be defined as coupled vibrations of the brake drum and two brake shoes.
Namely, as shown in FIG. 1, in a drum brake for a vehicle in which a pair of brake shoes 4 and 5 each having one end swingingly supported by a bracket 3 by way of a pin 2, for example, are disposed within a brake drum 1, while the other ends of the brake shoes 4 and 5 are opened by a cam 6 which is swung by a non-depicted actuator, such that linings 7 attached to the outer faces of the shoes 4 and 5 are pressed against the inner peripheral surface of the brake drum 1 so as to effect braking; when the above-mentioned "judder" occurs, the brake drum 1 vibrates in modes indicated by a continuous line 8 in FIGS. 4 and 5 (wherein FIGS. 4 and 5 respectively refer to the third- and fourth-order modes), whereas the brake shoes 4 and 5 vibrate in a mode indicated by a continuous line 9 in FIG. 6. Here, according to modal analysis, the vibration of the brake shoes 4 and 5 generates first-order twisting shown in (a) and (a') in FIG. 7 and first-order bending shown in (b) and (b') in the same drawing, as well as second-order twisting shown in (a) in FIG. 8 and second-order bending shown in (b) in the same drawing. The complex vibrations of the brake drum 2 and brake shoes 4 and 5 are coupled together, whereby coupled vibrations such as those shown in FIGS. 9, 10 (first-order mode), and 11 (second-order mode) are generated.
While FIGS. 12 and 13 show examples of brake shoes, the brake drum 1 as well as the brake shoes 4 and 5 have a number of natural frequencies at which the above-mentioned coupled vibrations are generated.
In each of the above-mentioned measures against "judder" 1) to 4), a natural frequency of the brake drum 1 or pair of brake shoes 4 and 5 is changed.
Since the brake drum 1 and the pair of brake drums 4 and 5 each have a number of natural frequencies as mentioned above, however, even when a certain natural frequency generating "judder" in the brake drum and/or pair of brake shoes is changed, the above-mentioned coupled vibrations may occur at other frequencies again and thereby generate "judder."
As a result of studies, the inventors have found that "the above-mentioned coupled vibrations are hard to cause at a plurality of natural frequencies including those of the brake drum and the like" when respective natural frequencies of a pair of right and left brake shoes, which are one of primary factors for causing the above-mentioned coupled vibrations, are made different from each other; and have associated this finding with the present invention. It is an object of the present invention to provide a drum brake for a vehicle in which vibrations generating "judder" are absorbed by a simple configuration and "judder" is less likely to occur than in the conventional drum brakes.
It is another object of the present invention to provide a drum brake for a vehicle which can be easily produced and can effectively suppress the above-mentioned "judder" by merely adding a simple processing to or changing the material of one or both of normal brake shoes having the same size.